The present invention is directed to devices that use plasma (very hot, ionized gases) for processing materials and for other industrial applications and, more particularly to a method and apparatus for cooling the chamber and chamber walls used to contain the plasma.
Plasmas confined within chambers are used to process materials in many industrial applications. For example, a toxic waste product may be processed by the plasma to convert the waste product into a nontoxic material. However, some waste products require very high temperatures in excess of 5,000xc2x0 F. to confidently decompose the waste product into nontoxic materials. This, in turn, requires careful selection of a material used to construct a chamber that can contain such a high temperature plasma and/or careful attention as to how the chamber is cooled. In some applications the plasma is allowed to contact (or cannot be prevented from contacting) the inner peripheral surface of the chamber which, in turn, causes erosion and/or corrosion of the inner peripheral surface of the chamber and release of impurities to the plasma. In these applications the chamber is replaced when the amount of erosion and/or corrosion exceeds a predefined limit. However, in very high temperature applications the heat generated by the plasma may be sufficient to destroy the chamber very quickly, so merely replacing the chamber is not a realistic option. Consequently, a proper method for cooling the chamber must be devised.
In magnetohydrodynamic (MHD) systems, a plasma is passed through a duct containing a magnetic field in order to generate an electrical current. In such systems it is known to form the duct walls with a porous material and then flow a gas through the porous duct walls to form a protective layer between the plasma and the inner peripheral surface of the duct wall. The protective layer may be used to protect electrodes, electrically conducting wall segments and/or electrically insulating wall segments from erosion, corrosion, evaporation or other deterioration. The protective layer flows with the plasma through the duct, thus helping to prevent contact between the duct wall and the plasma. While this technique is useful in relatively low temperature plasma flow systems, it is not as useful in systems that employ a very high temperature plasma, and especially not in systems that require the plasma to be confined within a chamber. Additionally, cool plasmas near the walls and electrodes of an MHD generator significantly reduce the generator efficiency. In some systems that employ a very high temperature plasma, radiation (e.g., x-ray, ultraviolet, infrared, etc.) may be emitted from the plasma. Such radiation is absorbed on or is partially reflected by the inner peripheral surface of the duct or chamber and eventually overheats the duct or chamber, or causes radiative assisted ablation of the chamber wall. Also, such plasma flow systems require the plasma to flow with the protective layer out of the duct or chamber.
Some low pressure plasma systems (P approximately equal to 10xe2x88x926 atmosphere) use magnetic fields to confine the plasma within the chamber. In these systems the plasma still tends to wobble or randomly move about the chamber and migrate toward the walls of the chamber due to fluctuating magnetic fields and turbulence in the rotating plasma. Computerized control of the magnetic field and energy densities helps to prevent wobbles in the plasma. However, as the plasma power levels and energy densities increase, the turbulence and increased plasma fluctuations cause the failure of the computer controlled magnetic field and the plasma touches the chamber wall. When the plasma touches the chamber wall, part of the chamber wall ablates and causes the plasma to move away from the wall. Of course, such ablation eventually requires replacement of the chamber. While such a magnetic field technique may work in lower energy density plasma systems with relatively slow deterioration of the chamber wall, any contact between the plasma and the wall in high energy density plasma systems could cause immediate destruction of the chamber.
In a plasma torch, a fine plasma arc is passed through a nozzle, and the plasma arc emerging from the nozzle is used to cut a material such as metal sheeting. During the cutting operation, hot molten or gaseous metal waste may impinge upon the nozzle and cause deflection of the plasma arc or a direct short to the metal. If the plasma arc contacts the inner nozzle wall, arcing and deterioration of the surface forming the nozzle orifice occurs, resulting in a reduced penetration rate, an increase in the roughness of the finished metal surface, and premature failure of the nozzle. One technique used to help stabilize the plasma arc is to flow a protective layer of a gas around the inner peripheral surface of the nozzle in a rotating vortex pattern. The vortex helps to contain and stabilize the plasma by providing a pressure containment and simultaneously imparting some rotation to the plasma jet. The cool protective layer helps to prevent contact between the plasma arc and the side of the nozzle. However, as with the use of a protective layer in MHD systems, this technique requires the plasma to exit with the protective layer.
Even in plasma systems that use a protective layer between the plasma and the chamber wall, the protective layer may not be adequate to fully protect the duct or nozzle wall when using very high temperature plasmas. One known method of accommodating higher temperature plasmas in a plasma torch is to use a liquid such as water to form the protective layer. The water undergoes a phase change from a liquid to a gaseous state as it is introduced into the nozzle. The phase change absorbs much more heat than is ordinarily possible using protective layers formed by gas alone, thus resulting in better cooling. However, as noted above, the techniques used in plasma torches also require the plasma to exit the nozzle with the protective layer.
The present invention is directed to a plasma processing system wherein a protective layer is formed between the plasma and a chamber in such a manner that the material forming the protective layer is allowed to exit the chamber without additionally requiring the plasma to exit the chamber. In one embodiment of the present invention, a plasma processing apparatus includes a first chamber having a first wall with an inner peripheral surface and an outlet. A plurality of fluid supplying outlets are disposed along the first wall and are configured to supply a cooling fluid into the first chamber that travels in a circumferential direction around the inner peripheral surface of the first wall and in a direction towards the outlet. The cooling fluid exiting the plurality of fluid supplying outlets forms a cooling layer for cooling the inner peripheral surface of the first wall, and the chamber is configured for allowing the cooling fluid to exit therethrough while substantially retaining the plasma therein.
In another embodiment of the present invention, a plasma processing apparatus includes a first chamber having a first wall with an inner peripheral surface and an outlet. A plasma is disposed in the chamber, wherein the plasma emits electromagnetic radiation. A plurality of fluid supplying outlets are disposed along the first wall and are configured to supply a cooling fluid into the first chamber to form a cooling layer for cooling the inner peripheral surface of the first wall. The chamber is configured for allowing the cooling fluid to exit therethrough while substantially retaining the plasma therein. A radiation communicating material is disposed at the first wall for communicating electromagnetic radiation from inside the first chamber away from the inner peripheral surface of the first wall. The radiation communicating material may communicate the electromagnetic material in a direction toward and/or away from the first chamber. If the plasma emits electromagnetic radiation having a wavelength in the ultraviolet range, then the radiation converting material may be of the type that absorbs electromagnetic radiation having a wavelength in the ultraviolet range and converts the absorbed electromagnetic radiation into electromagnetic radiation having a wavelength in the infrared range. If the radiation converting material is of the type which emits the converted radiation in all directions, and if the first wall includes a material that is transparent to electromagnetic radiation having a wavelength in the infrared range, then the infrared radiation passing through the first wall serves to avoid heating the first wall, while the infrared radiation emitted back toward the plasma helps to avoid unnecessary cooling of the plasma.
In another embodiment of the present invention, a plasma processing apparatus includes a first chamber having a first wall with an inner peripheral surface and an outlet. A plasma is disposed in the chamber, and a plurality of fluid supplying outlets are disposed along the first wall and are configured to supply a cooling fluid into the first chamber that forms a cooling layer for cooling the inner peripheral surface of the first wall. The chamber is configured for allowing the cooling fluid to exit therethrough while substantially retaining the plasma therein. A cooling fluid source provides the cooling fluid to the plurality of fluid supply outlets, wherein the cooling fluid has a structure that undergoes a phase change in response to a temperature in the cooling layer for providing increased cooling ability. In higher temperature applications, the cooling fluid may have a structure that undergoes disassociation and/or ionization to provide even more cooling ability.